Poseidon in Indonesia

• Approach:
  • Take catch-at-length data
  • Feed it into length-based assessment techniques
  • Estimate population/carrying capacity
  • Run realistic POSEIDON over it
• Advantages:
  • Best available science
  • Realistic
  • Can focus on details

Disadvantage

• Opaque:
  • How data affects model
  • What is important
• Slow:
  • Changes in assumptions/data require starting from scratch
• Uncertainty poorly defined

A possible workaround

• Keep your model simple
• Isolate parameter/state space by rejection sampling
• Focus less on:
  • The “right” parameters
  • The “right” distribution of parameters
• Focus more on:
  • a large ensemble of valid histories

Run the model “many” times

Start filtering

Keep filtering (SPR)

Keep filtering (Landings 2)

Keep filtering (SPR 2)

Simplified model

• 2 species (Lutjanus malabaricus + Pristipomoides multidens)
• 10x10 simple map
• 2 fleets (0-9; 10+)

Things we don’t know:

• Initial population
• Carrying capacity
• Current depletion and the like

Things we know:

• Last year landings
• SPR
• data we collected

Approach:

• Start the fishery “ab novo”
• Repeatedly:
  • Randomize all parameters \(K,L_{\inf},\dots\)
  • Run simplified POSEIDON for “many” years (up to 40)
    • Free entry
  • Accept simulation if model output are the same as what we observe in data
• Collect

Why bother?

• Benefits:
  • No need for population reconstruction
  • Clear uncertainty propagation
  • Clear value of information
• Cost:
  • Model needs to be fast

Results

• Ran the model for about 50,000 times
• Accepted about 1.5% of them
• This is their story….

Landings only scenario

• Imagine we only knew what this year landings (no length data)
  • 5000t to 10000t for malabaricus
  • 1000t to 3000t for multidens

Knowing SPR

• How does it change if I also add as a condition
  • OBSERVED SPR of malabaricus must be between 0.2% and 20% (measures from 2018 and 2016 but adding a bit of pessimism) respectively

Knowing SPR of other species

• How does knowing that the SPR of the other species is between 10 and 20 help?

Landings only (2)

Knowing SPR (2)

Knowing SPR of other species (2)

Basic results

• Once SPR is properly contextualized within the history of the fishery; these are actually green shoots
• Measuring the SPR of another species can increase our expected estimate of depletion for this species

Additional knowledge

1. I know that both boat populations are still at it
2. I know that small boats are more numerous
3. I know that malabaricus SPR has dropped dramatically?

Additional Knowledge 2

Additional Knowledge 3

Policy attempts

We don’t want just to assess, we want to know policy effects:

• no policy
• put a 100 day limit to 10GT only
• put a 100 day limit to all

Data-limited

• Works as before but:
  • apply same policy to all accepted runs
• Positive:
  • study the average effect
  • understanding the uncertainty
• Negative:
  • to make them comparable, speak only in relative terms

Results 1

Results Biomass

Results Landings

Results Cashflow

718

• 3 Species
  • Lutjanus malabaricus
  • Atrobucca brevis
  • Lethrinus laticaudis
• 3 Fleets
  • Large Longliners
  • Large Dropliners
  • Large Gillnetters

718 - Posteriors

718 - Policy

718 What’s missing

• Price increase
• Keep probolinggo boats very far
• Deal with seasonality of dropliners